This invention relates to an improvement in a conventional so-called continuous feeding method for controlled atmosphere storage (hereinafter referred to as "CA storage") including continuously feeding at a constant rate the air of a definite oxygen content (for example, 2 to 3%; the oxygen percent is by volume and the same applies hereinafter) into a storage room to maintain therein an atmosphere suitable for the storage. More particularly, this invention is to provide a CA storage method for vegetables and fruits, which, as compared with the conventional method, enables more rapid oxygen pulldown (rapid establishment of desired oxygen content in the atmosphere inside the storage room), more rapid increase to a desired level in the content of a gas component necessary for vegetables and fruits storage such as, for example, carbon dioxide, and, in addition, prevention of moisture dissipation from the storage room.
Various methods have heretofore been proposed for maintaining a storage room atmosphere at a composition suitable for the storage. These methods include (1) a common CA storage method which utilizes respiration of the vegetables and fruits themselves in effecting oxygen (hereinafter referred to as "O.sub.2 ") pull-down, removal of the excess amount of carbon dioxide (hereinafter referred to as "CO.sub.2 ") being effected by means of a CO.sub.2 scrubber and oxygen replenishment by supplying the open air through a fan, (2) a continuous feeding method, in which an artificially prepared air of a nearly the same composition as that of the desired storage room atmosphere is allowed to feed continually and at a constant rate throughout the storage period, (3) a combination of the common CA storage method and the continuous feeding method, in which the latter method is used at the start of the storage to rapidly reduce the O.sub.2 content and thereafter the same procedure as used in the former method is followed, and (4) a combustion method, in which the air withdrawn from the storage room is subjected to a combustion treatment and the resulting O.sub.2 poor air is returned to the storage room.
One of the techniques used in the above-said continuous feeding method for CA storage utilizes an adsorption and desorption apparatus (hereinafter referred to as "adsorber") containing an adsorbent to separate nitrogen (hereinafter referred to as "N.sub.2 ") and O.sub.2 in the open air by adsorption and desorption and the resulting air of the desired O.sub.2 content is fed continuously at a constant rate into a storage room to effect forced reduction in the oxygen content of the storage atmosphere, thus rapidly bringing the storage atmosphere conditions to a level suitable for CA storage (hereinafter such a continuous feeding method using said technique is referred to as "the conventional method").
In the conventional method, the open air is continuously supplied at a constant rate to the adsorber and the effluent air of the desired O.sub.2 content (about 2 to 3%) is continuously fed at a constant rate directly into the storage room to expel the inside air of a high O.sub.2 content (hereinafter such as air is referred to as an O.sub.2 -rich air), thus reducing the O.sub.2 content of the storage room atmosphere. Reduction in O.sub.2 content of the storage room atmosphere by such a procedure requires a long time, because the amount of air of the desired O.sub.2 content obtained from a definite amount of air supplied to the absorber is very small and the air of low O.sub.2 content (hereinafter such an air is refered to as an "O.sub.2 -poor air") of the desired O.sub.2 content is mixed with an O.sub.2 -rich air inside the storage room, the resulting mixture air being discharged from the storage room. Thus, assuming that the storage room is perfectly gas-tight and the storage room atmosphere is a uniform mixture, the rate of reduction in O.sub.2 content of the storage room atmosphere may be expressed by the following equation: ##EQU1## where t: time elapsed
V.sub.a : volume of the storage room PA1 V.sub.b : amount of air feed into storage room (per unit time) PA1 C: o.sub.2 content in % of the feed air C': initial oxygen content of the inside atmosphere of the storage room PA1 C": o.sub.2 content of the inside atmosphere of the storage room after the lapse of time t
The curve (I) in FIG. 1 is a graphic representation of the above equation.
As indicated by the curve (I), it is apparent that when the conventional method is used, with the decrease in O.sub.2 content of the storage room atmosphere, the rate of reduction in O.sub.2 content also decreases, meaning that the lower is the desired O.sub.2 content to be reached, the longer is the time required for the oxygen pull-down.
On the other hand, the conventional method has another disadvantage in that since CO.sub.2 increased by respiration of the vegetables and fruits is continually expelled from the storage room during the O.sub.2 pull-down, the CO.sub.2 content of the storage room atmosphere cannot increase above a value fixed (about 3%) according to the following equation: ##EQU2##
In the case of the above-said combustion method (4), in which the storage room atmosphere is subjected to a combustion treatment and recycled, the O.sub.2 pull-down is effected more rapidly, as compared with the above-said conventional method, but it is impossible to operate the method until a sufficiently low O.sub.2 content is reached, because as the O.sub.2 content of the storage room atmosphere is decreased, the amount of O.sub.2 necessary for the combustion becomes insufficient, resulting in increased formation of impurities due to incomplete combustion. The said method has further disadvantages of increased installation and running costs, as compared with the above-said conventional method.